How dangerous are induced currents in transmission towers and poles?

Electric Field

Landowners and developers should be aware of the potential for nuisance and startle shocks (caused by induced voltage) that can occur when someone comes in contact with a large conductive object (eg. a vehicle, building or even fencing) located on or off the right-of-way.

These shocks are known as ‘nuisance’ or ‘startle’ since they will not physically hurt someone, but will be noticed by some people and provoke a startle reaction.

How dangerous are induced currents in transmission towers and poles? (on photo: Hyderabad: Power lines, mostly high tension, pass dangerously near houses in colonies; credit: deccanchronicle.com)

These touch currents occur when a grounded individual touches an ungrounded object while standing in an Electric Field (e-field). Conductive objects when placed in an e-field attract a charge, and a person touching that object can experience an annoying or startling shock when a person’s body provides a path to ground for the current to follow.

There are many factors which influence the likelihood of nuisance shocks and the extent that people will notice them. These include:

Line voltage

Conductor to ground clearance

Size of the vehicle or object

Location on or off the right of way

Atmospheric conditions

Personal physiology

Electric fields emanate from any conductor or wire that carries voltage. Higher line voltages produce higher electric field strengths. The closer the conductor is to the ground, the higher the field strength beneath it tends to be, and the increased possibility of shocks.

Larger objects such as a building or a large vehicle have the capability of a larger charge and therefore the shock could be more noticeable. Backyard metallic objects such as swings, portable grills and lawnmowers have been known to deliver similar shocks.

Ungrounded metal wire fences can also receive sufficient charges to cause nuisance shocks. During building construction, workers have also received shocks when installing ungrounded gutters and downspouts on structures built close to the edge of the right-of-way. Homeowners may also experience shocks when cleaning gutters if the gutters are not properly grounded.

People or animals can receive a shock by touching a metal object located near a transmission line.

The shock is similar to that received by touching a television after walking across a carpet. The magnitude and the strength of the charge will be related to the mass of the ungrounded metal object and its orientation to the transmission line.

Induced current can be prevented or corrected by grounding metal objects near the transmission line.

Grounding chains can be installed on tractors. Metal fences can be connected to a simple ground rod with an insulated lead and wire clamp. Electric fences with proper grounding should continue functioning properly even when subject to induced voltage. Refueling vehicles directly under a high-voltage transmission line is not a good practice. A spark from a discharging metal structure with induced voltages to earth could ignite the fuel.

The risk of such ignition is higher with gasoline-powered vehicles than for diesel-powered vehicles.

Electric Induction – Structures

Typically, buildings and storage sheds will not be permitted within the utility corridor, and are not a particular concern. However, lower electric field strengths also can exist outside of the corridor and buildings outside of the corridor should be considered.

For structures outside the right of way, it is easy to reduce the potential for startle or annoyance possibilities by attaching a ground wire to the metal roof.

This protection also provides a measure of lightning protection for the structure. Buildings entirely made of metal are not normally of any concern because they are often inherently well grounded, but exceptions might exist for structures on wood foundations or on a high-resistance material. Again, it is usually a simple matter to ground such objects if necessary.

Zinc rain gutters

In the same manner, rain gutters on a large house fairly close to the transmission line could conceivably deliver a perceptible shock to a person on an aluminum ladder.

Quantitative worst-case analysis of any particular case can be difficult, but it is possible to determine if reducing the potential for perceptible shocks is a prudent precaution.

It is a simple procedure to attach a wire to the downspout and ground it to a metal water pipe or a driven ground rod to mitigate shock effects.

Electric Induction – Fences

Long fence wires that are strung on wooden posts can present shock possibilities if they run more or less parallel and close to the transmission line. The insulation quality of the wooden posts is the controlling parameter. Perpendicular fences will have significantly less induced current and voltage.

When exposed to the weather, even wooden posts are not perfect insulators. The lack of insulation will reduce the induced voltage on a fence wire and will limit the magnitude of a spark discharge.

Nevertheless, some fences could be insulated enough by the posts to make contact currents annoying when the fence is touched. This type of fence would have to be close to the line and quite long before it produced annoying currents. Long fences are often grounded by contact with growing vegetation. If grounded in this manner, electric field effects will be reduced, although magnetic field effects will remain unaffected.

The possibility of annoyance due to electric field induction could be eliminated by solidly grounding the fence at a single point, such as with a metal fence post.

For an “electric fence,” this is accomplished with a special filter designed to drain only the induced charge. However, a fence that is grounded at one or more points and otherwise insulated along a sufficient distance could present some opportunity for noticeable magnetic field induction effects.

Reducing the potential of these effects requires electrically breaking the fence into smaller grounded sections.

References:

Guidelines for Development Near Overhead Transmission Lines in BC – BC Hydro

Environmental impacts of transmission lines – Public Service Commission of Wisconsin

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About Author

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on Google+

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